To investigate the mutant white eye allele, Morgan mated the white-eyed male fly with a red-eyed (wild type) female. As shown in this figure, all of the F1 offspring had red eyes, indicating that the allele for white eyes was recessive. When Morgan bred the F1 generation flies to one another, he observed the classic 3:1 ratio of red:white eyes, however, only males had the mutant trait. All of the females and one-half of the males had the wild-type trait, which led Morgan to hypothesize that the gene that coded for eye color must be related to sex determination.
Female fruit flies have two X chromosomes (XX ) and males have one X and one Y (XY ). If the gene for eye color is located on the X chromosome, then this would explain why the trait rarely appears in females (i.e., a female would have to be homozygous for the recessive mutant). However, males would show the mutant trait if they have one copy of the allele because they have only one X chromosome and there is no corresponding gene for eye color on the Y chromosome. This study demonstrated that the gene for eye color in D. melanogaster was located on the X chromosome and provided further evidence for a chromosome theory of heredity. The X and Y chromosomes are called sex chromosomes because they determine the sex of an individual. Genes that are located on these chromosomes are called sex-linked genes . All other chromosomes in a cell (i.e., other than the sex chromosomes) are referred to as autosomes .
While there are several different systems for sex determination, for simplicity we will focus on the X-Y system of sex determination used by most mammals as well as some insects, snakes, fish, and plants. During meiosis, the two X chromosomes of females or the X and Y chromosomes of males pair together (although the X and Y are not homologous, they still pair during meiosis). One of these chromosomes then goes to each gamete, so females produce gametes with only X chromosomes, whereas males produce equal numbers of gametes with either an X or Y chromosome. If two gametes with X chromosomes undergo fertilization, the resultant offspring will be female (XX). That is, if an ovum with an X chromosome and a sperm with an X chromosome combine, the resultant offspring will be female. However, if an ovum with an X chromosome and a sperm with a Y chromosome combine, the offspring will be male (XY). While it has long been known that the X chromosome contains a substantial number of genes, researchers have only recently found genes on the Y chromosome, most of which are associated with the development of male gonads, including the sex determining region of the Y chromosome (SRY) gene in mammals. Therefore, it appears that a gene (or genes) on the Y chromosome provides the biochemical signal that begins the development of male gonads in embryos.
Not only do sex chromosomes determine the sex of an individual, but the X chromosome also has genes that code for many characters that are not related to sex determination. Sex-linked alleles in an XY chromosome system follow the patterns of inheritance observed in Morgan's studies, in which males transmit these genes to their female offspring, and females can transmit them to either male or female offspring. Females will only express recessive sex-linked traits that are homozygous, but males will express the trait coded for by their one X chromosome. As a result, sex-linked recessive traits are far more frequent in males than in females, although some females do exhibit sex-linked recessive traits. Since males have only one chromosomal location for sex-linked genes, the terms homozygous and heterozygous have little meaning. By convention, males are said to be hemizygous for sex-linked genes because they have half ("hemi") as many alleles as a female.
As with autosomal traits, you can use a Punnett square to study the transmission of sex-linked traits. When completing these crosses, the gametes identify the sex chromosomes only. The genes located on the X chromosome are indicated with superscript letters. (We will not examine any cases where we track a Y-linked gene.) As with Mendelian traits, a lowercase letter indicates a recessive allele, while an uppercase letter indicates a dominant allele. When discussing sex-linked traits, carriers (females who are heterozygous for a recessive trait) need to be considered.
Chromosome Behavior and Sex Chromosomes Part 1 VoiceThread Transcript